This invention relates to a system for the removal of metal poisons from fluidized catalytic cracking catalyst for converting hydrocarbons containing one or more metal poisons (vanadium or nickel).
One major operation in the modern refinery is the process of catalytic cracking. In this process, some of the heavier oils (often called xe2x80x9cgas oilsxe2x80x9d) produced upon fractionation of whole crude oil are decomposed or xe2x80x9ccrackedxe2x80x9d using fluidized zeolite-containing catalysts.
As the supply of light, sweet crude oils has dwindled during past years, catalytic cracking has become increasingly important in maintaining a supply of hydrocarbons suitable for use in various fuels such as gasoline. A problem that has occurred because of the increasing use of heavier, more sour crudes is that the heavier crudes contain substantially more organic metal compounds, such as vanadium and nickel porphyrins. These metals cause many undesirable reactions in heavy oil cracking in that the metals, specifically nickel and vanadium are quite harmful to the fluidized cracking catalysts used. These metals, present in the high-boiling fractions, deposit on cracking catalyst and accumulate with time. They act as poisons and have the resulting effect of increasing undesirable hydrogen and coke yields, decreasing the selectivity of the catalyst in making liquid products. It is also established that vanadium also attacks the zeolite itself, the high activity component of a catalytic cracking catalyst.
While the mitigation of Ni has been reasonably successful by using group V element additives, the solution for vanadium has remained elusive. The vast majority of prior art has dealt with the additive approach, the addition of so-called xe2x80x9ctrapping agentxe2x80x9d into FCC system either with the catalyst or with hydrocarbon feed. This passivation procedure may be seen in, e.g., U.S. Pat. Nos. 4,377,494 (barium); 4,451,355 (calcium); 4,473,463 (barium); and 4,824,815 (strontium). In addition to its obvious drawback of diluting catalyst activity, this approach of using additive trapping particles has suffered from lackluster V trapping performance. The reason is that the multitude of reactive conditions in FCC system interferes with the V trapping chemistry of most V traps. For example, it has been recognized that the presence of SOx in regenerator interferes with V-trapping chemistry. U.S. Pat. No. 5,324,416 teaches that V-trapping is more effective with the simultaneous use of SOx, transfer agents. This invention has a number of advantages over prior art: (1) no catalyst activity dilution; (2) it isolates V trapping from SOx and reducing environments, so that some effective V trap materials (Those that are also susceptible to SOx,) such as the divalent metal oxides can be used; (3) V trapping is done under conditions that can be tuned to optimize V transfer rate while keeping hydrothermal deactivation of zeolite catalyst in the vessel at minimum; (4) because the vessel may be simultaneously used as a catalyst cooler, it may allow handling of more Conradison carbon (con-carbon) in hydrocarbon feed; (5) depending on the concentration of V gathered by the trap, the V laden trap might be of value as source of vanadium metal.
Other V mitigation strategies include the use of V-trapping agent in isolated environment. U.S. Pat. No. 5,286,691 (Mobil) teaches the use of large V-trap particles which remain in the FCC regenerator (and is exposed to SOx, environment), and U.S. Pat. No. 5,260,240 (Chevron) teaches the use of V-traps in a external reactor. The V-ladden FCC catalyst is charged into the reactor and the operation is non-continuous.
The present invention is a fluidized catalytic cracking system which includes a fluidized catalytic cracking reactor, a conventional regenerator, and an outboard vessel containing a vanadium trapping material. Vanadium contaminated catalyst is circulated from regenerator to outboard vessel where the catalyst is fluidized with the V trap and vanadium is removed from the catalyst. Optionally the vessel may have features of a catalyst cooler to help lower the regenerator temperature.